1. Field of the Invention
The present invention relates to a one-dimensional or two-dimensional photoelectric transducer in which a semiconductor circuit including a plurality of photosensors using, for example, pn junctions is formed in the same semiconductor substrate. More particularly, the invention relates to a photoelectric transducer having a circuit configuration effective for reducing noise, such as fixed-pattern noise or the like.
2. Description of the Related Art
Recently, photoelectric transducers in which photosensors and a peripheral circuit are formed on the same substrate have been actively developed.
For example, a linear sensor in which an operational amplifier and photosensors are formed on the same semiconductor substrate (The Journal of the Institute of Television Engineers of Japan, Vo. 47, No. 9 (1993), pp. 1180), an image sensor having a sample-and-hold circuit (Japanese Patent Laid-Open Application (Kokai) No. 4-223771 (1992), and a solid-state image pickup device having an internal reference-voltage generation circuit configured by an operational amplifier (Japanese Patent Laid-Open Application (Kokai) No. 9-65215 (1997) have been proposed.
In general, a bias current for an operational amplifier is generated using a constant-current-source circuit. When forming such a constant-current-source circuit using MOS (metal oxide semiconductor) transistors, for example, a CMOS (complementary MOS) constant-current-source circuit as shown in FIG. 1 (R. Gregorian, G. C. Temes: Analog MOS Integrated Circuits for Signal Processing, p. 127, FIG. 4.5. ) is generally used. A CMOS constant-current-source circuit as disclosed in Japanese Patent Laid-Open Application (Kokai) No. 7-44254 (1995) has also been proposed.
The CMOS constant-current-source circuit shown in FIG. 1 is obtained by performing tandem connection of an upper-stage current mirror circuit and a lower-stage current mirror circuit between a power supply V.sub.DD and the ground. A constant-voltage-source output V.sub.01 is obtained from a connection point of the upper-stage current mirror circuit and the lower-stage current mirror circuit. That is, the upper-stage current mirror circuit includes a pMOS transistor Q4, whose source is connected to the power supply V.sup.DD, and a pMOS transistor Q3. The gates of the pMOS transistor Q4 and the pMOS transistor Q3 are connected to each other, and the gate and the drain of the pMOS transistor Q3 are directly connected, so that each drain supplies the load side with the same current I.sub.bias. The lower-stage current mirror circuit includes an nMOS transistor Q1, whose source is connected to the ground, and an nMOS transistor Q2 whose source is connected to the ground via a resistor R. The gates of the nMOS transistor Q1 and the nMOS transistor Q2 are connected to each other, and the gate and the drain of the nMOS transistor Q1 are directly connected, so that each drain obtains the same current from the load side. A constant current by the current mirror circuit passes through the resistor R. An output voltage provided by a voltage generated in the resistor R and the source-drain voltage of the nMOS transistor Q2 is output as the constant voltage V.sub.01 of this voltage source.
However, the inventors of the present invention have found that in conventional photoelectric transducers in which a CMOS constant-current-source circuit and photosensors are formed in the same semiconductor substrate, the CMOS constant-current-source circuit generates fixed-pattern noise.
When a voltage is applied to the gate of a MOS transistor, and a voltage is applied between the source and the drain in a state in which a channel is formed, carriers are accelerated by the concentration of the electric field near the end of the drain in the channel, to generate avalanche multiplication. Most of the carriers generated by the avalanche multiplication result in a substrate current. Light emission occurs in the process of recombination of such hot carriers. New electron-hole pairs are generated in the same semiconductor substrate by the emitted light. A part of the generated carriers diffuses through the semiconductor substrate and is mixed in the photosensors. Since the degree of mixture is not constant for all of the photosensors, fixed-pattern noise is generated.
FIG. 2 is a schematic diagram illustrating the manner of generation of fixed-pattern noise in a one-dimensional photoelectric transducer having a conventional CMOS constant-current source, together with a plan view of the semiconductor substrate. In FIG. 2, a constant-current-source circuit having the same configuration as the current mirror circuits shown in FIG. 1 is provided. The upper-stage current mirror circuit includes pMOS transistors, and the lower-stage current mirror circuit includes nMOS transistors. These circuits are disposed in the vicinity of the fourth to the sixth photosensor elements of a photosensor array. It can be understood that a dark output at a portion where the constant-current source is disposed is larger than other bit outputs and results in fixed-pattern noise.